Pyrotechnic delay charges are used in many fields, both military and civil, to provide a time delay between an inititating signal, for instance from an electrically activated fuse head or from a fuse, and triggering of a main reaction, such as ignition of a propulsive charge or triggering of a blasting charge. The charges will be described below in greater detail in relation to detonators for civil rock fragmentation.
A leading requirement for pyrotechnic delay charges is that they should burn with a well defined and stable burn rate having an insignificant time scatter. The burn rate should not be significantly influenced by the surrounding conditions or aging. Because of this, a burn having insignificant gas evolution and initial materials, intermediates and end-products with favourable and stable properties is required. The charges should be easily ignitable and provide good ignition transfer to other materials but without being excessively sensitive to jolts, vibration, friction or static electricity. The nominal rate should be adjustable with minor modifications of the charges. The charge compositions should be easy to prepare, dose and compress in safety. The charges should have a high energy content per unit weight and the incorporated components should not be too expensive.
Although conventional pyrotechnic elements can be said, in principle, to consist of a fuel and an oxidant, and therefore many substances should be usable, the above described requirements together significantly limit the choice of suitable initial materials. The component choice has come to be concentrated around a few established components within each field of application. For example, lead compounds are common ingredients in civil detonators.
Even though the amounts of pyrotechnic charge in the majority of initiator types are relatively small, there is a growing requirement that the charges should not contain toxic substances. This is in order to avoid problems during manufacture, to reduce emissions and also to lessen the problem of exposure at the point of end-use. It is also desirable that the preparation of the charges can be done without using solvents. Several components previously used in pyrotechnic elements are now no longer usable, for instance heavy metals.
A number of charges have been proposed with the goal of uniting good pyrotechnic properties with insignificant health consequences. For example, Swedish patent nos. 446 180 and 457 380 describe charges based on, inter alia, tin oxide as a principal non-toxic oxidant. However, these charges have less satisfactory properties as regards time adjustment and manufacture.